AC/DC converters may be widely used in many types of industry. For example, in many applications electrical power for components in a system may be provided using AC generators. Because certain types of components (or loads) may not use AC power, it would be common to rectify AC power to obtain DC power. Rectification, however, may not be accomplished with 100% efficiency, and typically results in reduction in power quality, for example, reduction in power factor and in increased harmonic distortions of the line currents. In many applications, the power quality may need to be at or above a certain level in order to prevent disturbance to other loads on the system.
For example, there is an increasing need for high power 3-phase AC/DC converters for the next generation of More Electric Aircraft (MEA). New standards for the power factor and, more importantly, the limits on the reflected current harmonics imposed by DO-160, Environmental Conditions and Test Procedures for Airborne Equipment, call for development of AC/DC converters, including those based on active power factor correction (PFC) topologies, that satisfy these stringent requirements.
Further, certain applications may have constraints on and/or requirements regarding the output power, weight, size, cost, complexity, and reliability, and on the electromagnetic compatibility (EMC) and electromagnetic interference (EMI) emissions of AC/DC converters.
There typically exist various other, often conflicting requirements on AC/DC conversion. Those may include, but are not limited to, requirements on different voltage conversion ratios (e.g., from 115 VAC to 270 VDC, or from 230 VAC to 540 VDC), “wild” frequency compatibility (i.e., the ability to convert AC power that varies in time in a wide frequency range, e.g., from 300 Hz to 800 Hz), output voltage ripple, soft start ability (e.g., for powering motors), over-current protection, cooling method(s), and environmental requirements/qualifications.
Power conversion technologies that are capable of meeting enhanced power quality requirements may be characterized as passive conversion, active conversion, and “hybrid” conversion employing harmonic correction techniques such as those based on harmonic injection and/or active filter implementation. An overview of different approaches to constructing three-phase rectifier systems may be found in, e.g., [3] and the references thereof.
For example, passive AC/DC power conversion may be accomplished with a plurality of diode pairs, where each pair is connected to a different phase of the AC input, to provide a rectified DC output. However, this type of AC/DC conversion may lead to substantial current harmonics that would pollute the electric power generation and distribution system. One solution to the foregoing problem may be to increase the number of supply phases for rectification.
To reduce current harmonics, multi-phase transformers (Transformer Rectifier Units, or TRUs) and/or autotransformers (Auto-Transformer Rectifier Units, or ATRUs) may be employed to increase the number of AC phases supplied to the rectifier unit. For example, in an 18-pulse passive AC/DC converter (18-pulse TRU/ATRU) the transformer/autotransformer may be used to transform the three-phase AC input whose phases are spaced at 1200, into a system with nine phases spaced at 40°. This would have the effect of reducing the harmonics associated with the AC/DC conversion.
Passive multi-phase harmonic reduction typically has the advantages of relative simplicity and low cost, absence of or reduced need for energy storage devices and/or control, high reliability (e.g., typical mean time between failures (MTBF) in excess of 100,000 hours), robustness (e.g., the ability to accepts high overloads), and low weight at high line frequencies (e.g., at 400 Hz and higher).
An absence of output voltage regulation may be viewed as the main disadvantage of passive conversion. Without such regulation, input voltage variations are proportionally passed to the output, and a change in the load would also result in a change in the output voltage. For example, in a passive converter from 115 VAC to 270 VDC a typical difference in the output voltage between no load to full load conditions may be approximately 4% to 6%, or 11 VDC to 16 VDC. To obtain output voltage regulation, an additional active DC/DC converter stage would need to follow a passive AC/DC converter stage, which would increase complexity, weight, and cost, and would decrease efficiency, robustness, and reliability of the converter. Also, the absence of regulation may result in presence of significant inrush currents.
To overcome certain limitations of passive conversion, various active conversion means may be employed, in addition or as an alternative to passive conversion. Such means may include, for example, harmonic correction techniques based on harmonic injection and/or active filter implementation.
As a main alternative to passive AC/DC converters, active (e.g., high frequency switch mode) AC/DC conversion may also be used, and such active conversion would be capable of providing regulated DC output voltage.
Output voltage regulation and/or adjustment may be considered the main advantage of active conversion. Also, active regulation topologies may operate in a wide range of line frequencies (e.g., the same active AC/DC converter may be used for 300 Hz, 800 Hz, and/or 50/60 Hz), and would typically have a built-in soft start ability, over-current protection, current limiting, and thermal protection. In addition, active AC/DC converters may have significantly lower weight for low AC frequency conversion in comparison with passive converters designed for such low frequencies.
The disadvantages of active converters may include higher cost and lower reliability, the need for high energy storage capacitor(s), and lower overload capabilities in comparison with passive conversion. In addition, active AC/DC converters may not easily accomplish conversion at some voltage conversion ratios.
For example, two typical active approaches may include boost (step-up) and buck (step-down) conversion. For a 115 VAC line-to-neutral 3-phase input voltage, a boost converter would be capable of providing ≧320 VDC output, while a buck converter would be capable of providing ≦230 VDC output. Thus the output voltage range from 230 VDC to 320 VDC may not be available without an additional DC/DC conversion stage.
Depending on the converter topology, the output power requirements, and the AC/DC voltage conversion ratios, practical state-of-art passive, hybrid, and active AC/DC converters would provide the input-current total harmonic distortion (THD) in a typical range of 3% to 12%. Further reduction of the THD (to, e.g., 1% to 2% range) may require more complicated multi-stage and/or multi-level approaches, with typically more than three active power switches, increased complexity and cost, and decreased robustness and reliability.
In addition, state-of-art active AC/DC converters targeting high power quality may require complicated regulation and control topologies, e.g., those utilizing variable switching frequency, fuzzy logic, and/or multiple feedback control loops.